Configure pre-build and post-build npm scripts

Right now I am not sure when dfx build call the npm build npm script. It would be nice to be able to provide pre-build and post-build scripts, because as part of your own build process you might want to deal with the generated code that dfx build gives you. I am not sure if I can rely on my build process running after dfx generates that code. This is related to complications describe here: Allow configuring the location of the .dfx generated code for asset canisters